This invention relates to a process for the catalytic conversion of xylenes containing ethyl benzene. More particularly, it is concerned with a process wherein xylenes containing ethyl benzene are contacted with a specific catalyst in vapor phase and in the presence of hydrogen to isomerize xylenes and convert the ethyl benzene into other aromatic hydrocarbons.
Among the xylene isomers, it is para-xylene and ortho-xylene that are industrially important at present. Para-xylene as a raw material of polyester synthetic fibers has been in increasingly great demand, and this tendency will not be changed also in future. Ortho-xylene is utilized as a raw material of phthalic acid ester, a plasticizer of polyvinyl chloride, but the present demand for ortho-xylene is less than that for para-xylene. On the other hand, meta-xylene finds little industrial application at present. Under the circumstances, it is important from the industrial point of view to convert meta- and ortho-xylenes into para-xylene.
Xylene isomers are close to one another in their boiling points, particularly the boiling point of para-xylene and that of meta-xylene are extremely close to each other, so it is disadvantageous from the economic point of view to separate para-xylene from the other xylene isomers by the distillation processing, for which reason the industrial separation of para-xylene has so far been conducted by the low temperature processing which utilizes the difference in melting point. In the low temperature processing, due to eutectic point, there is a limit to the recovery of para-xylene per pass, which is at most 60% per pass; as a result, the raffinate after recovery of para-xylene exhibits a fairly high concentration of para-xylene. There has recently been developed adsorption processing as a new separation technique as described in Japanese Pat. Nos. 17246/74, 28181/74, 10547/75, 11343/75 and 46093/76. According to the absorption processing, theoretically, para-xylene can be recovered 100% per pass; that is, the concentration of para-xylene in the raffinate after its adsorptive separation is very low, which is zero theoretically.
Ortho-xylene, in general, has so far been separated by the precision distillation processing.
The raffinate after separation of para- and ortho-xylenes is transferred to the isomerization process, where meta-xylene and/or ortho-xylene is isomerized up to the para-xylene concentration close to the thermodynamic equilibrium composition, then mixed with a fresh feed stock and transferred to the separation process, and this cycle is repeated. In such a combined process, when feeding to the isomerization process the raffinate after separation of para-xylene by the low temperature processing, as mentioned above, the concentration of para-xylene in the raffinate is relatively high, while the raffinate after separation of para-xylene by the adsorption processing exhibits a very low concentration of para-xylene. Consequently, a greater severity is required of the latter in the reaction in the isomerization process.
In general, xylenes utilized industrially are reformate xylenes obtained by reforming of naphtha and by subsequent aromatic extraction and fractional distillation, or pyrolysis xylenes obtained by aromatic extraction and fractional distillation of cracked gasoline by-produced from thermal cracking of naphtha. Pyrolysis xylenes are particularly characteristic in that the concentration of ethyl benzene is higher by two times or more than that of reformate xylenes. Table 1 below shows typical compositions of both reformate and pyrolysis xylenes.
TABLE 1 ______________________________________ Xylene Composition Component Reformate Pyrolysis ______________________________________ Ethyl benzene 18 wt. % 39 wt. % p-xylene 19 13 m-xylene 42 32 o-xylene 21 16 ______________________________________
Thus, in a mixture of xylene isomers there generally exist a fairly large amount of ethyl benzene, but unless ethyl benzene is removed by some means, it will become accumulated and its concentration will increase as the separation and isomerization processes are recycled, and thus the presence of ethyl benzene remaining unremoved results undesirably. For this reason, reformate xylenes containing less ethyl benzene are preferably utilized at present as a fresh feed stock. Anyhow, it is necessary to lower the concentration of ethyl benzene, and to this end there have been practised some methods industrially and some methods have been proposed. These methods are broadly classified into one involving direct separation of ethyl benzene and the other involving conversion by reaction of ethyl benzene into other useful compounds.
As the method of separating ethyl benzene, mention may be made of the distillation processing, and in this case it is necessary to carry out an ultra-precision distillation because of a small difference in boiling point between ethyl benzene and xylenes, whose industrial execution requires an enormous equipment investment and high running expenses, so the distillation processing is disadvantageous from the economic point of view.
Recently, as set forth in Japanese Patent Laid Open Publication No. 10223/77, etc., there has been proposed the adsorption processing for the separation of ethyl benzene, but its separation performance is not fully satisfactory.
As other means for removing ethyl benzene there have been proposed methods of converting it into other useful components, typical of which is the conversion of ethyl benzene into xylenes as is described in Japanese Pat. Nos. 46606/74, 47733/74, 15044/76, 36253/76 and Japanese Patent Laid Open Publication No. 16390/79. In this method, however, it is essential that platinum, a very expensive noble metal, be contained in the catalyst used.
In the conversion of ethyl benzene to xylenes, moveover, it is necessary from the standpoint of reaction mechanism that non-aromatic components such as naphthenes and paraffins be present therein, with their concentrations in the product reaching several percent to ten-odd percent. Furthermore, there is a limit to the conversion of ethyl benzene since it is restricted by thermodynamic equilibrium (see Table 2).
TABLE 2 ______________________________________ Equilibrium composition of xylene isomers (%) Temperature (.degree.K.) 600 800 1000 ______________________________________ Ethyl benzene 5.9 10.8 15.5 p-xylene 22.4 20.6 19.0 m-xylene 50.1 45.8 42.3 o-xylene 21.6 22.8 23.2 ______________________________________
A method of converting ethyl benzene into other components than xylenes has recently been proposed in Japanese Pat. No. 41657/78 and Japanese Patent Laid Open Publication No. 148028/77. According to this method, simultaneously with the isomerization of xylenes, ethyl benzene is converted to benzene and diethylbenzene by disproportionation reaction and by utilization of their great difference in boiling point from xylenes it is intended to separate them. The benzene thus obtained is in great demand as a raw material of the synthetic fiber nylon, but there is little demand for the diethylbenzene, which is necessary to be further converted into some other useful compound though this additional operation results in disadvantage economically. In addition, since the disproportionation reaction of ethyl benzene is restricted by thermodynamic equilibrium, there is a limit to the conversion of ethyl benzene.
In view of the above circumstances, it is keenly desired at present to provide a method capable of converting ethyl benzene into a useful compound without being restricted by thermodynamic equilibrium. Furthermore, as will be apparent from what has been mentioned hereinbefore, there exists a keen demand for the development of a catalyst system having characteristics permitting an effective use of even xylenes having a high concentration of ethyl benzene, e.g. pyrolysis xylenes, and, in the isomerization of xylenes, capable of sufficiently isomerizing even raffinate after adsorptive separation of para-xylene.
In view of such present situations, we have considered, for the removal of ethyl benzene, a catalyst system permitting the hydrodealkylation of ethyl benzene into benzene under little influence of thermodynamic restrictions and ethane and the simultaneous isomerization of xylenes; as a result, having reached the conclusion that this reaction can be performed by combination of a solid acid component and a hydrogenation component, we made extensive studies about various catalysts.